1. Field of the Invention
The present invention relates to solar power production, and more particularly to solar boilers and solar receiver panels for solar boilers.
2. Description of Related Art
Solar power generation has been considered a viable source to help provide for energy needs in a time of increasing consciousness of the environmental aspects of power production. Solar energy production relies mainly on the ability to collect and convert energy freely available from the sun and can be produced with very little impact on the environment. Solar power can be utilized without creating radioactive waste as in nuclear power production, and without producing pollutant emissions including greenhouse gases as in fossil fuel power production. Solar power production is independent of fluctuating fuel costs and does not consume non-renewable resources.
Solar power generators generally employ fields of controlled mirrors, called heliostats, to gather and concentrate sunlight on a receiver to provide a heat source for power production. A solar receiver typically takes the form of a panel of tubes conveying a working fluid therethrough. Previous solar generators have used working fluids such as molten salt because it has the ability to store energy, allowing power generation when there is no solar radiation. The heated working fluids are typically conveyed to a heat exchanger where they release heat into a second working fluid such as air, water, or steam. Power is generated by driving heated air or steam through a turbine that drives an electrical generator.
More recently, it has been determined that solar power production can be increased and simplified by using water/steam as the only working fluid in a receiver that is a boiler. This can eliminate the need for an inefficient heat exchanger between two different working fluids. This development has lead to new challenges in handling the intense solar heat without damage to the system. In a solar boiler, heat transfer rates can reach levels around 2-3 times the heat transfer rate of a typical fossil fuel fired boiler. This high heat transfer rate intensifies problems related to maintaining even heating and flow distribution throughout known designs of boiler panels. The high heat transfer rate gives rise to high pressures and temperatures in the boiler tubing and related structures. Additionally, in solar boilers, only one side of each boiler tube is heated, while the opposed side is shaded, which gives rise to stresses related to gradients in thermal expansion/contraction. Moreover, since the sun rises and sets each day, solar boilers must undergo a daily thermal expansion/contraction cycle, which can lead to an increase in creep and fatigue damage from the cycling stresses.
In typical boilers, for example coal fired boilers, the heat transfer surfaces, namely the superheater, reheater, and evaporator, are separated with respect to physical location as well as the mode of heat transfer, e.g., convection versus radiation dominant. The separation of heat transfer surfaces is important because each different section contains fluid with different physical properties including temperature, pressure, and quality. This separation allows the different heat transfer surfaces to be individually supported and allows for different thermal expansion rates in each of the different heat transfer surfaces. For example, the evaporative section in a typical boiler takes the shape of a quadrilateral, with rigid supports on the centerline of each wall and fixed welds on each corner. This is possible because each side of the boiler will expand at a constant and equal rate, since each side has the same steam conditions and experiences similar heat flux. When the boiler heats up and grows, the quadrilateral evaporative section expands without adding additional stress on the evaporative panels.
In contrast, solar boilers often have heat transfer surfaces that are integral, adjacent, or stacked on top of one another, meaning the heat transfer surfaces are within close proximity to one another. Moreover, each individual heat transfer surface can have extremely different heat flux inputs, steam temperatures, and metal temperatures, due to the uneven distribution of solar energy. If a traditional method of supporting the panels is used, as in traditional coal fired boilers, for example, the large variation in thermal expansion would cause the panels to pull themselves apart.
While the known systems of solar power production have generally been considered satisfactory for their intended purposes, there has remained a need in the art for solar boilers and solar boiler panels that can better accommodate the heat and stress related to solar power production. There also has remained a need in the art for such solar boilers and solar boiler panels that are easy to make and use. The present invention provides a solution to these problems.